1. Field of the Invention
The present invention relates to a color image processing apparatus for reproducing a color image.
2. Description of the Prior Art
In a prior art color image processing apparatus of this type, when a color image is to be read and reproduced, a text image is read by a scanner which decomposes it to red, green and blue component color images which are primary colors in an additive coloring system. Then, the component color images are processed by complementary color conversion, masking or undercolor removal to produce color image data of yellow, mazenta, cyan and black which are three primary colors in a substractive coloring system. The color image data are combined to produce the color image.
The color processing is performed prior to the combination of the color image data because inks of the respective component colors used to reproduce the color image, for example, yellow, magenta and cyan ink have spectram reflection factor characteristics which are different from ideal ones.
In the undercolor removal processing, undercolors of equal amount which form black are removed from the yellow, mazenta and cyan image data, and the black quantity (or black component) is calculated to obtain image data corresponding to a black. The processing is necessary to precisely reproduce gray which is hardly reproduced precisely by the three colors, yellow, mazenta and cyan. It is important in a printing technique that the same amount of inks are used in changing the three primary color inks into the black ink.
In the undercolor removal processing, a minimum density of the yellow, mazenta and cyan color component densities is set to 100% and undercolors of equal amount less than the minimum density are removed from the respective color component densities and the undercolors of the equal amount less than the minimum density represent the black quantity. The amount of undercolor removal in a normal color image processing is less than 50% of the minimum density, and a shadow area is reproduced by the black quantity calculated by the undercolor removal and the black formed by mixing the three remaining primary colors. If all of the minimum density (100%) were used as the black quantity a highlight area would appear too dark. In the prior art color image processing apparatus which reproduces the shadow area by the black produced by the undercolor removal processing and the black produced by the combination of the three colors, a full color image such as a color photograph which is colorful and includes gray areas can be reproduced by multi-color printing without any serious problems. However, when a color image text which includes the full color image and a monochromatic binary image such as black characters, or a detail area including fine lines drawn on a shadow area is to be read and reproduced, a problem is encountered. Since the black in the character or detail area is reproduced by superposition of the three colors and the black produced by the undercolor removal processing, the color becomes muddy and the edges of the characters or the detail areas become dim.
The black reproduction by the superposition of colors raises a problem of color offset. In an ink jet recording system which has recently been used as a color image recording system or a high speed recording system such as an electrophotographic system or thermal transfer recording system, the color offset by the superposition of colors raises a problem. When a text image which includes black characters in a half-tone color image is to be read and reproduced, the color offset of yellow, mazenta, cyan and black at the edges of the black characters is remarkable and the color of the characters becomes muddy or dim.